The invention relates to a multi-cylinder dryer having upper drying cylinders and lower drying cylinders over which a paper web to be dried is run in meandering fashion from one cylinder onto the nest cylinder. The paper web is in direct contact with each drying cylinder within a sector that is larger than 180.degree.. The dryer includes an upper wire and a lower wire arranged to press the paper web against the faces of the drying cylinders. The wires are guided by guide rolls, which are placed to support the paper web continuously by a wire as the web runs from one cylinder to the next cylinder.
In a further aspect, the invention concerns a multi-cylinder dryer, having several sequential cylinder groups, of which the cylinder groups in a former group employs a single-wire draw or twin-wire draw, and wherein a latter group employs a twin-wire draw. In the wire draws, the wires are arranged to guide the paper web to be dried via the drying cylinders, the wires being guided by guide rolls.
As it is known in prior art, a multi-cylinder dryer of a paper machine comprises two lines of drying cylinders placed one line above the other, the web being guided in meandering fashion over said cylinders. In a multi-cylinder dryer, different cylinder groups are employed, such as wire groups and drive groups. In a latter one of the groups power of rotation of the cylinders is passed to all the cylinders belonging to the same group by means of a common drive shaft and a gear transmission.
In recent years, the running speeds of paper machines have been increasing constantly, and now the limit of 1500 m/min is approaching. As a result, fluttering of the web and detaching of the web from a support fabric constitute a serious problem that hampers the running quality of a paper machine.
As is known in prior art, in multi-cylinder dryers, so-called twin-wire draw is employed, wherein the paper web has free gaps when it moves from one cylinder line to the other. Moreover, especially in the initial part of a multi-cylinder dryer, so-called single-wire draw is employed, wherein the drying wire is arranged to support the web also when the web moves from one cylinder line to the other. In such case, on one line of cylinders, the drying cylinders remain inside the loop of the drying wire, and on the other line of cylinders they remain outside the loop.
The present invention is applied in particular in connection with so-called twin-wire draw, which, in the present application, means a mode of supporting and drawing the web in connection with heated drying cylinders wherein on the upper cylinders an upper wire is used and on the lower cylinders, in a corresponding way, a lower wire is used. The wires are guided by faces of the drying cylinders and by transfer rolls placed in the spaces between the cylinders. Thereby, on the upper line of cylinders, the web is pressed into direct drying contact with the face of an upper cylinder and, in a corresponding way, the web is pressed by the lower wire against the face of a lower cylinder.
In a twin-wire draw, the web has usually had substantially long open passages as it runs from one cylinder line onto the other. These open passages have been susceptible to fluttering breaks and wrinkles in the web, which drawback has become more serious with increasing running speeds of the paper machines. The drawback has been manifested with particular emphasis in the initial part of the drying section, where the web is still relatively moist and therefore, of low strength, and where its elastic properties encourage fluttering. Attempts have been made to eliminate this drawback by making the open draws of the web shorter in the initial part of the drying section, and by placing imaginary planes passing through the axes of the upper and lower cylinder lines at a shorter distance from one another than that which has been usually customary or would be optimal, e.g. in view of the efficiency of drying.
Conversion of a 3rd and a 4th drying group to a single-wire draw has also been used. But this is an emergency solution, because it results in reduced evaporation capacity and makes it more difficult to arrange the air-conditioning.
Attempts have been made to solve the problems of running quality in the drying section, resulting from increased running speeds of the paper machines, by moving over to single-wire draw. Since this lowers the drying capacity of the machine, this solution is, as a rule, not advisable, except where it is necessary, i.e. in the first and the second drying group. In the other groups, in a drying section, attempts are made to operate with twin-wire draw, which is preferable in view of the drying capacity.
One prior art solution for twin-wire draw is described in the U.S. Pat. No. 3,250,019, wherein the object is to provide a fully closed draw. In this prior art solution, the wire of the preceding group accompanies the paper web up to the suction zone of the following wire. However, there is a drawback in the sense that the paper web cannot be brought tightly into a pocket formed by the cylinder and the wire; excessively long free gaps remain for the web.
Another prior art solution is described in the DE Patent No. 36 23 971, wherein the object is also to provide a fully closed draw. There is, however, a drawback that excessively long free gaps remain for the paper web, with resulting fluttering.
With the use of the prior art arrangements, the draw of the paper web in the gaps between the groups of cylinders in a multi-cylinder dryer has become problematic. If an open draw of the web is applied in the group gaps, it causes fluttering of the web with resulting risk of break of the web and inferior running quality.
Moreover, in prior art, in the group gaps, it has been necessary to use quite a large difference in speed, which stretches the web detrimentally.
In ways known in prior art, attempts have been made to solve the above problems of the draw of the web in the group gaps in a multi-cylinder dryer by various means, such as overlapping of the wire guide rolls, whereby it has been attempted to close the open draw of the web in the group gaps. As further aid, various blow boxes have been used. By means of these solutions, it has however, not been possible to avoid all the problems. As an additional drawback, more difficult threading of the web has been experienced.
One prior art solution is described in the FI patent application No. 871493. In this prior art solution, the web is passed from one group to the other by means of a particular group gap cylinder. However, such a group gap cylinder has a large size, and it has no heating effect, so that passing of the web over a group gap cylinder does not promote the drying of the web. Moreover, when such a large diameter group-gap cylinder is used, the spaces between the groups become congested.